This invention relates to a control circuit for a voltage controlled variable attenuator, and more particularly to a control circuit for a variable attenuator which is adapted to be used as an electronic volume control device in an acoustic apparatus.
A typical example of a voltage controlled variable attenuator is shown in FIG. 1. This example is in the form of a differential amplifier having bipolar transistors Q.sub.1 and Q.sub.2, the emitters of which are commonly connected. A current source outputting a current I.sub.1 is connected between the commonly connected emitters and a reference voltage--B, and the current I.sub.1 is controlled in accordance with a control signal. More specifically, a control voltage V.sub.C is applied as a differential input of the differential amplifier for controlling its attenuation, and an attenuated output created at a collector resistor R.sub.l of the transistor Q.sub.1 is delivered at an output terminal OUT of the differential amplifier.
The voltage controlled varible attenuator of the above described construction exhibits an I.sub.2 /I.sub.1 (dB) versus control voltage V.sub.C (mV) attenuation characteristic as shown in FIG. 2. Since the input-output transfer characteristic, that is, the characteristic between the base-emitter voltage V.sub.BE and the collector current I.sub.C of the differential transistors Q.sub.1 and Q.sub.2 is not linear, the output current I.sub.2 is expressed as EQU I.sub.2 =I.sub.1 /{1+exp (qV.sub.C /kT)} (1)
wherein q represents the electric charge of an electron, k represents the Bolzmann constant, and T represents absolute temperature. Accordingly, when the control voltage V.sub.C varies in a linear manner, the output attenuation I.sub.2 /I.sub.1 (in dB) expressed in the form of a logarithmic value also varies in a linear manner as shown in FIG. 2.
A variable attenuator of the above described attenuating characteristics would exhibit a somewhat unnatural auditory sensation when used as an electronic volume control device in an acoustic apparatus. Generally speaking, it is desirable that the volume control device have an operational characteristic exhibiting a large gain variation versus control voltage in its high attenuation range, while exhibiting a small gain variation versus a control voltage in its low attenuation range. The operational characteristic of the circuit shown in FIG. 1, however, is as shown in FIG. 2, which shows a substantially linear variation in the attenuation of the control voltage throughout the entire range of operation, thus rendering an unnatural auditory sensation.
Furthermore, the input and output transfer characteristics of the bipolar transistors Q.sub.1 and Q.sub.2 are varied according to temperature variations of the transistors, and therefore the attenuation characteristics of the circuit shown in FIG. 1 are varied according to temperature as shown by a dashed line (for 75.degree. C.) and an x-dash line (for -25.degree. C.), such a feature constituting a drawback of the conventional circuit.